The present invention relates to circuits for driving and braking a controllable D.C. motor.
German patent application No. P 25 57 280.0 discloses a circuit for driving and braking of a speed-controlled direct current motor, particularly for the drive of a vehicle such as a trolley car. The motor is energized during driving operations by a direct current source by means of a choking coil and a direct current regulator. Braking is provided by a braking resistance and a first diode connected in parallel with the field winding of the motor. Switches are provided to carry out the necessary connection of components to perform driving or braking operations. During the driving operation, the field winding, in series with a switch, is connected in shunt with the armature of the motor. During the braking operation, the braking resistance in series with the switch is connected in shunt with the armature of the motor. Between the quenching diode of the direct current regulator and one terminal of the resistance a second diode is provided, and during braking operations a third diode is connected between one terminal of the field winding and one terminal of the choking coil. This circuit provides a combination of regenerative and dynamic braking operations for a D.C. motor, depending on the absorbing capability of the power supply network. The regenerative braking operation requires a wide range of settings, especially at the lower end. However, in case of the circuit described, the limit of the lower range of settings is primarily controlled by the value of the voltage across the reversing condenser of the direct current regulator. There is the possibility that the available margin of settings within this range of operations will be reduced by an irregular, intermittent pulse rate. The resultant unstable operation of the regulator can lead to a temporary interruption of the current fed into a trolley network.